BRUEL & KJAER 1623 - Filter Sweep Rate Limitations

An exception to this rule occurs with the filter resting in Preset at a frequency be-
tween 2Hz and 200Hz and with the noise filter switched in, if a tuning signal of above
200Hz is applied the filter will only track onto it if the tuning signal level is high enough
to compensate for the attenuation of the filter at the tuning frequency. This need not be
a
problem because the Preset frequency needs only to be set above 200Hz, for exam-
ple, by setting the MANUAL TUNING/PRESET slide switch to the 200Hz to 20kHz posi-
tion. As a general rule one should always try to pre-tune the 1623 with the MANUAL
TUNING/PRESET knob, to the frequency one expects the tuning (tracking) signal to start
at .
When using the noise filter in conjunction with the Photoelectric Tachometer Probe MM
0012, best triggering is obtained when the generated waveform is approximately sym-
metrical. This is achieved by making the reflective and non-reflective portions of the ro-
tating part approximately equal in width.
The noise filter is of special value when performing harmonic analysis on audio equip-
ment using the Frequency Response Test Unit Type 441 6 and the Test Records OR
2009 and OR 2010. In this application, high frequency record noise and the 1 kHz syn-
chronisation tone on the records are automatically filtered out by the noise filter at low
tuning frequencies. A suitable setting of the TRIGGER LEVEL knob for this application is
approx. + 0,4 V or -0,4 V.
4.8. FILTER SWEEP RATE LIMITATIONS
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As is the case with all electronic filters, a certain time is required for the filter to settle
at the tuned frequency. In practice this means that if the 1623 is swept across a fre-
quency range too rapidly, with either the external tuning signal or the manual tuning
control, the filter will not be given enough time to up-date (settle). This can result in sig-
nificant errors in the filtered signal level.
With reference to the chart shown in Fig.4.8 it can be seen that the settling time of the
filter is a function of frequency, with the longest time needed for the lowest frequency.
The fastest possible sweep while maintaining specified filter accuracy (with < -1 dB ad-
ditional error) is attained with a hyperbolic sweep in accordance with Fig.4.8. The mini-
mum time required for performing such a sweep between two frequencies can be found
from this chart. With a linear or logarithmic sweep the filter sweep rate is determined by
the lowest tuned frequency, but as the sweep progresses up in frequency the sweep
rate could be progressively increased.
With a logarithmic sweep, the initial sweep rate in decades/minute, again determined
by the lowest tuned frequency can also be found directly from Fig.4.8.
A further factor which limits the frequency sweep rate is the maximum sweep rate of
the Level Recorder Type 2306 which, in combination with the 1623 is 30 decades/min-
ute.
When tuning the filter from a tachometer signal from a rotating machine it will be diffi-
cult to accurately assess the rate at which the machine speed is changing. It should be
borne in mind that higher frequencies and wider bandwidths make the situation less criti-
cal.